1. Field of the Invention
The present invention pertains to a system and method for ventilating a patient that allows a ventilator to be connected to the airway of a patient using either a single-limb breathing circuit or a dual-limb breathing circuit to optimize the flexibility of the ventilator and treatment options available for the patient.
2. Description of the Related Art
It is known to provide pressure support to a patient through a single-limb breathing circuit and a patient interface. Such ventilation systems typically generate a flow of gas using, for example, a blower. The pressure or flow of gas delivered to the patient is controlled by controlling the operating speed of the blower, controlling a valve that diverts gas from the gas flow path, or a combination thereof. The gas flow is communicated to the airway of the patient by means of the breathing circuit, which is usually a flexible tube coupled to a gas outlet port of the ventilator. The distal end of the single-limb breathing circuit includes the patient interface, such as a nasal mask, nasal/oral (full) mask, total face mask that covers the face, or nasal canula, that couples the breathing circuit to the airway of the patient.
An exhaust port is provided in the breathing circuit, the patient interface, at a coupling between the breathing circuit and the patient interface, or any combination thereof. The exhaust port allows gas to pass from the interior of the breathing circuit or patient interface to the ambient atmosphere. A typical exhaust port consists of a relatively small, fixed geometry orifice or a plurality of orifices. Variable geometry orifices are also known. During exhalation, some of the patient's expired gas passes to atmosphere via the exhaust port, and some expired gas flows back up the breathing circuit to toward the ventilator. It is possible, depending on the size of the expiration, for expired gas to exit the gas system through the pressure control valve in the ventilator.
This technique for exhausting gas through the exhaust port, and possibly through the pressure/flow control valve in the ventilator inspiratory limb, is advantageous in that there is a relatively low resistance to the expiratory flow. Thus, the expiratory effort (work) exerted by the patient during exhalation is minimized in ventilators that use a single-limb circuit to communicate the flow of gas to the patient. In addition to providing ventilation using a mask, single-limb ventilators, which are also referred to as pressure support systems, are used to provide various modes of pressure support, including, for example, continuous positive airway pressure (CPAP) support, bi-level pressure therapy that varies the treatment pressure with the user's respiratory cycle, proportional assist ventilation (PAV®) that varies the pressure with respiratory effort, proportional positive airway pressure (PPAP) ventilation that varies the pressure with flow or with a predetermined profile, auto-titration pressures support that varies the pressure based on a monitored condition of the user.
Critical care ventilators provide support to a patient using a dual-limb circuit that includes an inspiratory limb and an expiratory limb. Such ventilators are often referred to as “invasive” ventilators because the patient interface is typically a device inserted into the airway of the patient. However, it is known to use a non-invasive patient interface in a dual-limb ventilator. A flow of gas is typically generated by a compressor, blower, piston, or bellows. The inspiratory limb carries the flow of gas from the ventilator to the patient, and the expiratory limb carries the flow of gas from the patient to an exhaust valve, which is typically provided within the ventilator. The exhaust valve controls the flow of exhaust gas from the system, i.e., from the expiratory limb. The proximal portions of both the inspiratory limb and the expiratory limb are coupled to the ventilator, and the distal portions of both the inspiratory limb and the expiratory limb are connected to a Y-connector near the patient. The patient interface in a dual-limb ventilator, which is coupled to the Y-connector, is typically a tracheostomy tube or an endotracheal tube. However, it is known to use a nasal/oral mask to interface the breathing to the patient, so long as the leakage of gas from the mask is minimized.
It can be appreciated that one difference between a single-limb ventilator configuration and a dual-limb ventilator configuration resides in the ability of the ventilator to manage leaks. In a single-limb ventilator system, there is a known leak from the system through the exhaust port as well as potential unknown leaks, such as leaks at the mask/patient interface. Techniques are known and employed in a single-limb ventilator system to account for the known leak, as well as the unknown leaks, to ensure that the patient receives the desired pressure and/or flow. Thus, single-limb ventilator systems are also referred to as “leak tolerant” systems. In a dual-limb ventilator system, however, the system is closed, meaning that there are not supposed to be leaks, intentional or otherwise, so that the ventilator can precisely control the pressure, volume, and/or flow of gas delivered to and expired from the patient. Thus, dual-limb ventilator systems are not leak tolerant as they do not have the ability to account for leaks.
Because a dual-limb ventilator system has the ability to control the pressure, volume, and/or flow of gas delivered to the patient with greater accuracy than a single-limb ventilator system, dual-limb ventilator systems are better suited for use in life support situations. Conversely, dual-limb ventilator systems are not well suited to situations where non-invasive ventilation is desired, because of their inability to handle leaks, which are common when ventilating a patient non-invasively. In addition, when a conventional dual-limb ventilator system is used in a non-invasive ventilation mode, it is typically still used in a dual-limb configuration. However, this dual-limb configuration is disadvantageous because the presence of the expiratory limb results in a relatively high expiratory resistance (resistance to exhalation), which is typically higher than that present in conventional single-limb ventilator systems. This high expiatory resistance is due to the fact that the expiatory flow must pass through the entire expiratory limb, as well as through flow sensors, bacteria filters, and the exhaust valve.
It is also known to provide non-invasive ventilation in a critical care ventilator by providing a single-limb circuit having a proximal end coupled to the inspiratory limb portion of the ventilator and a distal end coupled to the patient interface. Conventional ventilators having this configuration use an actively controlled exhaust valve provided at the distal end of the single-limb circuit. In addition, a hardwired connection must be provided between the ventilator and the actively controlled valve, so that the ventilator can control the operation of the valve. More specifically, the valve is controlled to open (exhaust gas to atmosphere) during an expiratory phase of a breathing cycle and close during the inspiratory phase. This configuration is disadvantageous because it requires the relative bulky and cumbersome actively controlled valve to be “hung” from the distal end of the single-limb circuit, i.e., the location where the Y-connector would be in a dual-limb circuit. In addition, the hardwired connection to the actively controlled valve presents entanglement issues with the wire connection.